Enveloped viruses enter cells either by fusing their envelope directly to a host cell membrane or by fusing to endosomal membrane after being internalized. Fusion in the first entry pathway is triggered by binding to cognate receptor(s), whereas fusion in endocytotic pathway is initiated by low pH within the endosome. Strong evidence exists that the Env glycoprotein of the retrovirus, avian sarcoma and leukosis virus (ASLV), uses both receptor binding and low pH to enter the cell: it undergoes initial conformational changes at the plasma membrane as a result of receptor binding, and, after endocytosis, undergoes final changes induced by low pH within endosomes. The unprecedented utilization of this dual triggering mechanism allows the sequential refolding of the ASLV Env that leads to fusion to be better delineated than possible for other viruses. The low pH-requirement for fusion between cells expressing ASLV Env and cognate receptor-expressing cells has been unambiguously demonstrated, but the requirement for low pH during virus entry is still controversial. The pH-dependence of virus-cell and virus-liposome fusion will be rigorously investigated, using lipid and content mixing assays. The identity of the pH-sensitive steps of ASLV Env-induced fusion will be determined by arresting sequential stages of fusion and testing whether low pH is required to proceed to the subsequent stage. For many unrelated viral proteins, a common structural motif referred to as a six-helix bundle, has emerged; there is compelling evidence that this structure is critical for fusion. The sequence homology between ASLV Env and other fusion proteins with known structures strongly indicates its propensity to form a six-helix bundle. Whether ASLV Env does fold into a bundle to promote fusion will be tested by determining if Env-derived synthetic peptides abolish cell-cell fusion, as was the case for other viral proteins that fold into bundles. Delineating the mechanism of ASLV Env-induced fusion will provide insight into the basic principles by which important human pathogens, such as HIV, Ebola, and flu viruses enter cells and could therefore suggest new antiviral strategies against viral pathogens.